Arup Samanta

Photoluminescent silicon quantum dots in core/shell configuration: synthesis by low temperature and spontaneous plasma processing.

Quantum confinement in zero-dimensional silicon nanocrystals (nC) in the quantum dot (QD) configuration has triggered a tremendous interest in nanostructured device technology. However, the formation of Si-QDs eventually proceeds through multi-step routes and involves high temperature processing that impedes preferred device configuration. The present work demonstrates the formation of nC-Si QDs of controlled size, density and distribution through one-step and spontaneous plasma processing, at a low substrate temperature (300 °C) compatible for device fabrication. Direct growth of nC-Si/SiO(x) core/shell quantum dots embedded in the a-Si matrix, 6.4-3.7 nm in diameter and with number density in the range ∼ 6 × 10(9)-1 × 10(11) cm(-2) has been accomplished, following a novel route where He dilution to SiH(4) in RF plasma CVD has been found instrumental. On gradual reduction in the size of QDs, splitting of the energy bands widens the optical band gap and induces visible photoluminescence that appears controllable by tuning the size and density of the dots. This low temperature and spontaneous plasma processing of nC-Si/SiO(x) core/shell QDs that exhibit the quantum size effect in photoluminescence is being reported for the first time.

Transport spectroscopy of coupled donors in silicon nano-transistors

The impact of dopant atoms in transistor functionality has significantly changed over the past few decades. In downscaled transistors, discrete dopants with uncontrolled positions and number induce fluctuations in device operation. On the other hand, by gaining access to tunneling through individual dopants, a new type of devices is developed: dopant-atom-based transistors. So far, most studies report transport through dopants randomly located in the channel. However, for practical applications, it is critical to control the location of the donors with simple techniques. Here, we fabricate silicon transistors with selectively nanoscale-doped channels using nano-lithography and thermal-diffusion doping processes. Coupled phosphorus donors form a quantum dot with the ground state split into a number of levels practically equal to the number of coupled donors, when the number of donors is small. Tunneling-transport spectroscopy reveals fine features which can be correlated with the different numbers of donors inside the quantum dot, as also suggested by first-principles simulation results.

Optical, electrical and structural properties of SiO : H films prepared from He dilution to the SiH4 plasma

The effect of incorporation of oxygen into the initially crystalline-like Si : H network and its gradual increase up to 20 at% on the optical, electrical and structural properties of the material has been studied systematically. The prime objective of the experiment was to investigate the contribution of He as the diluent to the SiH4 plasma in modifying the SiO : H network when CO2 was used as the source of oxygen, and to extract some ideas that might be extended in the future development of nc-SiO : H network from a similar plasma with suitably modified parameters in RF-PECVD. Incorporation of oxygen invariably widened the optical gap; however, it reduced the electrical conductivity by several orders of magnitude with a significant increase in its activation energy. A sharp increase in the bonded H content (CH) of the film was identified at the initial stage of oxygen incorporation and that occurred mostly due to the gross change in the network structure from the crystalline-like to an amorphous-like phase. However, on further addition of oxygen CH gradually decreased, bonded mostly in the polyhydride configuration and the overall film surface roughness diminished. Systematic reduction in CH identified a dehydrogenation process occurring in the Si network during the gradual inclusion of oxygen, induced by the presence of He as the diluent to the plasma. The result seems opposite to the conventional H2-diluted plasma and appears significantly favourable for the future development of nc-SiO : H materials under optimized plasma parameters.

Effect of RF power on the formation and size evolution of nC-Si quantum dots in an amorphous SiOx matrix

The evolution of silicon quantum dots (Si-QDs) embedded in a-SiOx matrix has been controlled by varying the RF power in the He-diluted SiH4 plasma at a low substrate temperature of 300 °C in PECVD. By optimizing the plasma parameters, Si-QDs of very small size, down to ∼2 nm in diameter, with extremely narrow size dispersion at FWHM ∼1 nm and a very high density of ∼2 × 1012 cm−2 has been obtained. The effect of applied RF power controlling the growth mechanism of Si-QDs has been explained by using probable plasma chemistry of SiH4 in He, inside the plasma. Si-QDs with such low dimension and with high density, embedded in a-SiOx matrix, are being reported for the first time. SiOx is the most preferred dielectric medium in device fabrication. This finding by direct plasma synthesis at low substrate temperature may provide strong impetus to the further development of optoelectronic and photonic devices based on Si-QDs.

Comparative study of donor-induced quantum dots in Si nano-channels by single-electron transport characterization and Kelvin probe force microscopy

We comparatively study donor-induced quantum dots in Si nanoscale-channel transistors for a wide range of doping concentration by analysis of single-electron tunneling transport and surface potential measured by Kelvin probe force microscopy (KPFM). By correlating KPFM observations of donor-induced potential landscapes with simulations based on Thomas-Fermi approximation, it is demonstrated that single-electron tunneling transport at lowest gate voltages (for smallest coverage of screening electrons) is governed most frequently by only one dominant quantum dot, regardless of doping concentration. Doping concentration, however, primarily affects the internal structure of the quantum dot. At low concentrations, individual donors form most of the quantum dots, i.e., “donor-atom” quantum dots. In contrast, at high concentrations above metal-insulator transition, closely placed donors instead of individual donors form more complex quantum dots, i.e., “donor-cluster” quantum dots. The potential depth of these “donor-cluster” quantum dots is significantly reduced by increasing gate voltage (increasing coverage of screening electrons), leading to the occurrence of multiple competing quantum dots.

Tunneling in Systems of Coupled Dopant-Atoms in Silicon Nano-devices.

Following the rapid development of the electronics industry and technology, it is expected that future electronic devices will operate based on functional units at the level of electrically active molecules or even atoms. One pathway to observe and characterize such fundamental operation is to focus on identifying isolated or coupled dopants in nanoscale silicon transistors, the building blocks of present electronics. Here, we review some of the recent progress in the research along this direction, with a focus on devices fabricated with simple and CMOS-compatible-processing technology. We present results from a scanning probe method (Kelvin probe force microscopy) which show direct observation of dopant-induced potential modulations. We also discuss tunneling transport behavior based on the analysis of low-temperature I-V characteristics for devices representative for different regimes of doping concentration, i.e., different inter-dopant coupling strengths. This overview outlines the present status of the field, opening also directions toward practical implementation of dopant-atom devices.

SiOx nanowires with intrinsic nC-Si quantum dots: the enhancement of the optical absorption and photoluminescence

We report a very simple method for the fabrication of amorphous silicon rich oxide nanowires (a-SiOx NWs) with intrinsic nanocrystalline silicon quantum dots (nC-Si QDs) with a high density and controlled size, by thermally annealing thin films of nC-Si quantum dots embedded in the a-SiOx matrix, overcoated by an ultra-thin Au catalyst layer. The size and density of the Si-QDs in the basic matrix are controllable by changing the plasma parameters during the growth of the nC-Si/a-SiOx thin films. The diameter of the grown SiOx nanowires is controlled by merely varying the thickness of the Au-coating. The formation of the 1D NWs with rigid boundaries influences a shrinkage in the size of the intrinsic 0D QDs; the size-reduction of the QDs is more prominent in narrower NWs obtained from thinner catalyst layers. This unique quantum-dot/nanowire composite system demonstrates a significantly improved optical absorption and reduced reflection in the entire UV-visible range of the solar spectrum compared to its thin film structure, and those are again pronounced by reducing the dimension of the nanowires. The photoluminescence properties of this composite system demonstrate a strong room temperature emission band in the range of 400–600 nm with a peak at ∼519 nm. The PL peak of the NW film undergoes a large blue shift by 150 nm from that of the annealed QD films which has been attributed to the reduction in the average size of the intrinsic nC-Si QDs during the growth of the a-SiOx NWs. These novel a-SiOx NWs with intrinsic nC-Si QDs, produced by a simple synthesis technique involving a solid–liquid–solid (SLS) growth process, deserve enough promise in the field of photovoltaics and light emitting devices.

Effect of selective doping on the spatial dispersion of donor-induced quantum dots in Si nanoscale transistors

In donor-atom Si transistors, donor-induced potential minima work as quantum dots (QDs) giving rise to single-electron tunneling peaks at low temperatures. Until now, however, current–voltage (I–V) characteristics have not been directly correlated with measured donor-induced potentials. For that, we study the spatial dispersion of potential minima by Kelvin probe force microscopy combined with potential simulations and electrical characteristics. As a result, for low doping concentrations, current peaks are ascribed to spatially scattered QDs, whereas for high doping concentrations with a selective doping pattern, peaks are ascribed to donor-cluster QDs located at less-scattered centralized position due to a macroscopic U-shaped potential, suggesting a way for QD position control.

Single-electron quantization at room temperature in a-few-donor quantum dot in silicon nano-transistors

Quantum dots formed by donor-atoms in Si nanodevices can provide a breakthrough for functionality at the atomic level with one-by-one control of electrons. However, single-electron effects in donor-atom devices have only been observed at low temperatures mainly due to the low tunnel barriers. If a few donor-atoms are closely coupled as a molecule to form a quantum dot, the ground-state energy level is significantly deepened, leading to higher tunnel barriers. Here, we demonstrate that such an a-few-donor quantum dot, formed by selective conventional doping of phosphorus (P) donors in a Si nano-channel, sustains Coulomb blockade behavior even at room temperature. In this work, such a quantum dot is formed by 3 P-donors located near the center of the selectively-doped area, which is consistent with a statistical analysis. This finding demonstrates practical conditions for atomic- and molecular-level electronics based on donor-atoms in silicon nanodevices.

Electric-field-assisted formation of an interfacial double-donor molecule in silicon nano-transistors

Control of coupling of dopant atoms in silicon nanostructures is a fundamental challenge for dopant-based applications. However, it is difficult to find systems of only a few dopants that can be directly addressed and, therefore, experimental demonstration has not yet been obtained. In this work, we identify pairs of donor atoms in the nano-channel of a silicon field-effect transistor and demonstrate merging of the donor-induced potential wells at the interface by applying vertical electric field. This system can be described as an interfacial double-donor molecule. Single-electron tunneling current is used to probe the modification of the potential well. When merging occurs at the interface, the gate capacitance of the potential well suddenly increases, leading to an abrupt shift of the tunneling current peak to lower gate voltages. This is due to the decrease of the system’s charging energy, as confirmed by Coulomb blockade simulations. These results represent the first experimental observation of electric-field-assisted formation of an interfacial double-donor molecule, opening a pathway for designing functional devices using multiple coupled dopant atoms.